1. Field of the Invention:
The present invention relates to a motor-driven flow rate control valve device suitable for an EGR (Exhaust Gas Recirculation) control system.
2. Description of the Prior Art:
FIG. 4 shows one example of a conventionallly proposed step motor valve designed for controlling the EGR control. A housing 41 includes an inlet 42 communicating with an exhaust system of an engine and an outlet 43 communicating with an admission system thereof. Exhaust gas discharged from the engine passes from the inlet 42 through a reflux passage 44 and further flows from the inlet 42 to the admission system, thus recirculating the exhaust gas. A valve 45 is so provided in the reflux passage 44 as to be capable of impinging upon a seat member 46. The amount of recirculation of the exhaust gas flowing through the reflux passage 44 is regulated by this valve 45.
A valve shaft integral with the valve 45 is so disposed as to be swayable in the axial direction with the help of a bearing 49 which is installed through the intermediary of a holder member 48. To be specific, the valve 45 assumes a so-called poppet valve structure wherein it is driven in an open direction with the result that separation from the seat member 46 is made. To the right end portion of the valve shaft 47 is fixed a spring holder 50 for holding a return spring 51. The valve 45 is constantly forced to move to a closing position by the return spring 51, viz., in a direction in which the valve 45 can impinge upon the seat member 46.
On the other hand, a motor housing 53 is joined through an insulator 52 to the right end of the housing 41. The motor housing 53 accommodates a step motor 54. This step motor 54 is capable of controlling the rotation in forward and reverse directions at rotary angles step by step in response to electric signals transmitted from a computer, and this rotary motion is converted into a linear motion by means of an internal screw 55. Videlicet, when the step motor 54 is rotated leftwardly under predetermined pulse control in the case of its being viewed from the direction indicated by an arrowhead B, the motor shaft 56 is moved by the internal screw 55 in the direction pointed by the arrowhead B, thereby propagating the driving force which acts in the open direction via the valve shaft 47 to the valve 45. At this time, since a load of installation of the return spring 51 is set so as to be small as compared with the driving force of the step motor 54, opening of the valve 45 can be controlled, overcoming the spring force of the return spring 51. It is to be noted that the reference numeral 57 stands for a cooling air layer.
The above-mentioned computer converts a car speed indicating an operating condition of the engine the number of rotation thereof, a temperature thereof and an aperture of a throttel into electric signals, and then transmits them to the step motor 54. The step motor 54 is capable of controlling the rotationin the forward and reverse directions step by step in response to the electric signals, and it is therefore feasible to effect position control of the motor shaft 56, i.e., the axial position control of the valve shaft 47 in a highly accurate manner. Consequently, the amount of EGR can surely be controlled with high accuracy in accordance with the operating condition of the engine.
The above-described conventional mechanism is, however, characterized by a disadvantage in that the valve 45 is locked as it remains open in the event of disconnection or a motor problems such as damage in the screw mechanism, which causes problems such as inferiority both in drivability and in emission.